Introduction

Platelets have been considered mostly for their role in hemostasis, thrombosis and vascular disease; however, many studies over the last few decades have provided an emerging body of evidence on their potential role in host defence reactions. This unconventional function of platelets is consistent with the theory that platelets may represent the phylogenetic vestige of the primitive hemocytes of lower organisms. As hemocytes, which exert a central role both in defence against invading microorganisms and in repair of tissue injury, platelets maintain in many regards the function of inflammatory cells, able to activate complement, secrete proteases, alter vascular permeability and tone and express cytotoxic properties. For instance, platelets have been shown to participate in cytotoxicity against the parasites responsible for shistosomiasis, toxoplasmosis, and trypanosomiasis.

Although platelets are anucleated cells, they have an anatomical structure and biochemical properties in many aspects similar to leukocytes and it is thus conceivable that platelets may participate in the defence against infections and in other inflammatory responses. For instance, several observations have confirmed the involvement of platelets in allergic disease and in particular in the tissue inflammatory changes associated with allergic asthma. However, given the role of paramount importance that platelets play in hemostasis and thrombosis and considering the complicacy of the mechanisms regulating these functions, which have absorbed much of the research efforts on these cells, little attention has been paid to the mechanisms regulating platelet inflammatory activities. This chapter will discuss functional activities of platelets related to chemotaxis, an important step in cell involvement in tissue inflammation.